(1) Field of the Invention
This invention relates to electronic equipment for processing signals, particularly analog signals, such as those in the audio and video range, into corresponding digital signals suitable for recording on a record medium such as magnetic recording tape, and particularly relates to systems for correction of errors when such digitized signals are played back and optionally reconverted into an output analog signal.
(2) Description of the Prior Art
The preparation of commercial recordings, both audio and video generally begins at the point where a number of tracks, such as 4, 8 or 16 tracks of analog audio signals are recorded onto a master tape, the master tape is then mixed with other masters to dub in other signals and the mixed masters are further mixed into sub-master tapes having a single monaural track, two stereo tracks or four quadraphonic tracks. The sub-master tapes are then used as the basis for manufacture of both record discs and prerecorded tapes. Any defects or deficiencies inherent in the analog signals are thus embodied in the subsequent generation master and sub-master recordings and become a part of all manufactured copies. In order to greatly reduce, if not completely eliminate many such deficiencies, professional recorders are now widely considering the adaptation of digital recorders such as are generally used in instrumentation and computer data processing fields for audio and video uses. In such recorders, as for example that depicted in U.S. Pat. No. 3,786,201 (Myers et al), the analog signals are periodically sampled and a digital word is generated corresponding to each sample. Since the signal to tape interface affects only the fine structure digit carrying signal and not the numerical content per se, the integrity of the digitized audio signal is maintained and no degradation in the quality of the recorded sound occurs, even with repeated re-recording, mixing and the like. Typical decreases in the amplitude or pulse rise time, etc. of the digital pulses may be recovered by conventional signal processing techniques.
However, despite the inherent desirability of such digital audio and video recorders, general acceptance in the professional recording industry has not yet been achieved. It is believed that at least part of the reason for the lack of acceptance has been the propensity of errors in the digitized signal as may result from defects in the record medium such as the familiar drop-out problem in magnetic recording tape. Rather than merely causing a momentary loss of signal as in conventional recorders, the loss of a digital bit may, if it occurs at a most inopportune time, cause the signal to lose sync entirely such that all subsequent portions of the digital signal are meaningless. To avoid such total loss, it is conventional to group the digitized data words formed from a number of bits into blocks or frames, each of which is indexed by a sync word. Such systems still do not prevent the loss of data within a given frame, which loss will still result in an undesirable shift in the output level or other disturbing noises as well as the actual loss in the intended sounds.
In order to prevent the loss of computer or other data processing information, systems have been developed for detecting the presence of errors in a playback signal and for correcting errors so detected. Typically, such data processing recorders enable error correction by providing redundant information which may then be recovered and played back in the event an error in a primary track is discerned. Most simply, such systems provide two (or more) totally redundant data tracks and record the same information on each of the tracks. Particularly, the data on the two tracks may be desirably spatially staggered along the length of the tape such that a single defect spanning both tracks will not cause the loss of the same portion of the signal. While such fully redundant systems are technically feasible, they obviously require twice as much record medium as would otherwise be needed. More sophisticated recorders have also been devised in which error correction codes are generated and recorded along with the digital data, such that when an error is detected, the correction codes are decoded to regenerate a corrected data portion corresponding to the erroneous data. Such schemes also generally utilize multiple tracks, in which one or more tracks may be exclusively devoted to storage of the error correcting code (ECC). See Patel, U.S. Pat. No. 3,745,528. In that patent, the error detection operation provides error pointers (pointing to an erroneous block of data) which are generated by determining the quality of the playback signal, i.e., overall waveforms, etc.
However, not all data or recording systems are amenable to multitrack recording. Particularly, to facilitate compatibility with previously accepted recorder systems for use in audio, video and high density digital recorders, it is desirable to provide a single track digitized recorder in which an error correction operation is also employed. In U.S. Pat. No. 3,913,068 (Patel), a single track recorder is disclosed which utilizes a data format in which error check codes are included at the end of a block of data and in which external indicators are detected to initiate the need for error correction.